Are you talking about trimming it down to a different size or adding bends?
If you are trimming it you should be able to do it yourself. Use some kind of hand saw like a coping saw. DO NOT use a power tool because it generates too much heat and will end up melting the plastic instead of cutting it. Also use tape (on both sides) to keep the edges from chipping. After you do the cut then use plenty of water and fine sand paper to dress up the edge then use plastic polish to finish it off.
If you are talking about adding bends I would suggest checking the yellow pages to see if there is a company near you that works with Lexan. They will probably have the tools needed to heat and bend it without inducing any distortions.